|Publication number||US7105373 B1|
|Application number||US 10/640,963|
|Publication date||Sep 12, 2006|
|Filing date||Aug 14, 2003|
|Priority date||Aug 14, 2003|
|Publication number||10640963, 640963, US 7105373 B1, US 7105373B1, US-B1-7105373, US7105373 B1, US7105373B1|
|Inventors||Peter J. Hopper, Philipp Lindorfer, Vladislav Vashchenko, Andy Strachan|
|Original Assignee||National Semiconductor Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (30), Non-Patent Citations (1), Referenced by (7), Classifications (10), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
As further shown in the
The n+ regions 112 and the p+ regions 114 vertically alternate and contact each other such that each p+ region 114, except for the bottom p+ region 114, lies between and contacts an upper n+ region 112 and a lower n+ region 112. The bottom p+ region 114, In turn, contacts an upper n+ region 112.
The thicknesses of the n+ regions 112 and p+ regions 114 are selected to minimize the likelihood that the charge carriers from photogenerated electron-hole pairs will recombine. Further, the total thickness of photodiode 100, which is equal to the total number of n+ regions 112 and p+ regions 114 that are utilized, is selected based on the wavelengths of the photons that are to be collected during the operation of photodiode 100.
In addition to n+ and p+ regions 112 and 114, photodiode 110 also includes a p+ sinker 120 and an n+ sinker 122. P+ sinker 120 and n+ sinker 122 have substantially uniform dopant concentrations. Further, p+ sinker 120 is formed in semiconductor material 110 so that p+ sinker 120 contacts each p+ region 114, while n+ sinker 122 is formed in semiconductor material 110, spaced apart from p+ sinker 120, so that n+ sinker 122 contacts each n+ region 112. As a result, n+ regions 112/n+ sinker 122 and p+ regions 114/p+ sinker 120 form a single interdigitated p-n junction.
Further, photodiode 110 includes a layer of isolation material 124 that is formed over the top n+ region 112, a first metal contact 130 that is formed through isolation material 124 to make an electrical connection with n+ sinker 122, and a second metal contact 132 that is formed through isolation material 124 to make an electrical connection with p+ sinker 120.
In operation, a single imaging cycle includes a reset period, an integration period, and a read out period. During the reset period, photodiode 100 is reset by applying a positive reset voltage to the n+ regions 112 via metal contact 130 and n+ sinker 122, while p+ regions 114 are connected to ground via metal contact 132 and p+ sinker 120.
The positive reset voltage reverse biases the interdigitated p-n junction (without breaking down the p-n junction), and sets up an electric field that attracts all or substantially all of the free electrons from the p+ regions 114 to the n+ regions 112 and n+ sinker 122 where the electrons are collected. As a result, the p+ regions 114 are depleted or substantially depleted of electrons.
At the end of the reset period, the positive reset voltage is removed. When the positive reset pulse is removed, a positive charge remains on the n+ regions 112 and n+ sinker 122 as there is no discharge path for the positive charge to follow. As a result, the electric field remains after the reset voltage has been removed.
As noted above, an integration period follows the reset period. During the integration period, photodiode 100 collects the photons that strike photodiode 100. When a photon strikes photodiode 100, the photon penetrates a distance into semiconductor material 110, depending on the wavelength of the photon, where the energy of the photon creates an electron-hole pair.
For example, blue photons (from blue light) tend to form electron-hole pairs in a blue region near the surface, whereas red photons (from red light) tend to form electron-hole pairs in a red region that is much deeper in semiconductor material 110. Green photons (from green light), on the other hand, form electron-hole pairs in a green region that lies between the blue and red regions, while infra-red (IR) light forms electron-hole pairs in an IR region that lies below the red region.
When electron-hole pairs are formed in the p+ regions 114, the holes diffuse over to p+ sinker 120 where they are collected by the ground connection to second metal contact 132. The electrons, on the other hand, diffuse over to the n+ regions 112 and n+ sinker 122, where the electrons reduce the magnitude of the positive charge that was left on the n+ regions 112 and n+ sinker 122 when the reset voltage was removed.
When electron-hole pairs are formed in the n+ regions 112, the holes diffuse over to the p+ regions 114 and p+ sinker 120 and on to metal contact 132. At the same time, the electrons formed in the n+ regions 112 reduce the magnitude of the positive charge that was left on the n+ regions 112 and n+ sinker 122.
Thus, at the end of the integration period (a predetermined period of time), the magnitude of the positive charge on the n+ regions 112 and n+ sinker 122 that was present after the reset voltage was removed has been reduced by the number of photons that were collected during the integration period.
Following the integration period, during the read out period, the magnitude of the positive charge that is left on the n+ regions 112 and n+ sinker 122 is sensed by a sensing circuit, such as an active pixel sensor cell or a charge coupled device (CCD). Thus, based on the change in positive charge over the integration period, the number of photons received by photodiode 100 can be determined.
One of the advantages of the present invention is that photodiode 100 utilizes a number of highly doped n-type and p-type regions 112 and 114, which are sized to minimize recombination, to form the interdigitated junction. The highly doped n-type and p-type regions provide a relatively low resistance path for the photogenerated carriers to follow.
In addition, the highly doped n-type and p-type regions 112 and 114 allow a relatively large reset voltage to be used. A relatively large reset voltage, in turn, generates a relatively strong electric field. Thus, once the reset voltage has been removed, a relatively strong electric field remains.
By providing a low resistance path and a strong electric field, the present invention allows photogenerated carriers to be quickly extracted from the interdigitated p-n junction. Thus, although photodiode 100 has a large overall capacitance, a low resistance path and a strong electric field provide for relatively short photocarrier transit times which, in turn, provide fast performance.
Another advantage of the present invention is that the total thickness of photodiode 100 can be made to be relatively large, e.g., five microns, by utilizing a larger number of n+ and p+ regions 112 and 114. By forming photodiode 100 to be relatively thick, photodiode 110 is more sensitive to visible light towards the red end of the spectrum as well as IR light. Thus, the present invention provides a photodiode that is both fast and sensitive to red and IR wavelengths.
As further shown in
For example, a first gas that includes boron can be introduced during the epitaxial growth for a first predetermined time to form a p+ region 214. The first gas is removed after the first predetermined time, and followed by the introduction of a second gas that includes, for example, phosphorous. The second gas is present during the epitaxial growth for a second predetermined time to form an n+ region 216. Thus, by altering the gas that is introduced during the epitaxial growth, the p+ and n+ regions 214 and 216 can be formed.
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It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.
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|U.S. Classification||438/60, 257/465, 438/74, 438/75|
|Cooperative Classification||Y02E10/547, H01L31/103, H01L31/028|
|European Classification||H01L31/028, H01L31/103|
|Nov 24, 2003||AS||Assignment|
Owner name: NATIONAL SEMICONDUCTOR CORPORATION, CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:HOPPER, PETER J.;LINDORFER, PHILIPP;VASHCHENKO, VLADISLAV;AND OTHERS;REEL/FRAME:014715/0857;SIGNING DATES FROM 20031001 TO 20031002
|Dec 5, 2006||CC||Certificate of correction|
|Mar 12, 2010||FPAY||Fee payment|
Year of fee payment: 4
|Feb 25, 2014||FPAY||Fee payment|
Year of fee payment: 8